Question: Simplify the following expression: $z = \dfrac{1}{10r} - \dfrac{1}{8r}$
Answer: In order to subtract expressions, they must have a common denominator. The smallest common denominator is the least common multiple of $10r$ and $8r$ $\lcm(10r, 8r) = 40r$ $ z = \dfrac{4}{4} \cdot \dfrac{1}{10r} - \dfrac{5}{5} \cdot \dfrac{1}{8r} $ $z = \dfrac{4}{40r} - \dfrac{5}{40r}$ $z = \dfrac{4 -5}{40r}$ $z = \dfrac{-1}{40r}$